Field of the Invention
Pressure sensitive adhesive (“PSA”) compositions comprising polymer having a photoinitiator bound to the polymer chain that comprises at least one vinyl group. The PSA is cured by exposure to ultraviolet (“UV”) radiation. The combination of the bound photoinitiator and vinyl group provides a synergistic effect with respect to UV curing. The cured PSA is useful for industrial tapes, and may be especially useful in tape applications for medical, food or electronics markets.
The Related Art
PSA comprising bound Type II photoinitiator groups have been discussed in the art, an example of which are BASF acResin® products. Type II photoinitiators involve bimolecular hydrogen abstraction based on species such as benzophenone or thioxanthone. Due to radical-radical coupling, PSA crosslinking reactions are possible without the need for vinyl groups. A limitation with such adhesives concerns depth of cure due to filtering of the UV light by the bound photoinitiator such that effective cure typically can only be attained with adhesive film thicknesses less than about 2 or 3 mils. Another limitation with Type II photoinitiators, particularly benzophenone, is that such species have been shown to be only effective at about 256 nm, and many additives can also absorb in this region and compromise UV cure.
The free radicals generated from a Type I photoinitiator, that is cleavage type photoinitiators, are known to be more reactive due to shorter lifetimes. With reactive double bonds present, these types of photoinitiators can be very effective at inducing UV cure. Also, they can be designed to have a broader UV absorption than typical Type II photoinitiators, such as benzophenone. Depending on the substitutions on the aromatic ring, some Type I photoinitiators absorb well above 256 nm and a few are even up into the visible spectrum (greater than 390 nm). Thus, a photoinitiator can be chosen that has absorption outside of the absorption window of other aromatic species in an adhesive composition, such as aromatic monomers, inhibitors and tackifiers. This is not always possible when Type II photoinitiators are used. A disadvantage with Type I photoinitiators concern the potential for migration from the adhesive, which is a significant concern if the adhesive is used in applications where migration of unused photoinitiator from the post cured adhesive is a concern.
UV curable PSA having good depth of cure with limited migration of photoinitiator species and photoinitiator compatibility with other components of the adhesive composition is desired. Such PSAs are particularly sought for applications where migration of the photoinitiators is a concern, such as in medical applications, electronic applications and foods.
All parts and percentages set forth in this specification and the appended claims are on a weight-by-weight basis unless otherwise noted.